1. Field of the Invention
The present invention relates to a method for under-color removal in a color image forming apparatus converting separated color signals B, G and R into corresponding output signals for pigments Y, M and C through color conversion means, and outputting them.
2. Discussion of the Related Art
In a digital copying machine, analog signals generated by reading originals are converted into digital gray-scale values to carry out an image quality adjusting process for adjusting granularity, degradation, fineness, etc. and are recorded and reproduced in dot patterns. In particular, because the gray-scale values are stored digitally, various amendments and editing of the data are easily carried out in memory as well as image data processing for generating an image with high definition and reproducibility.
In a full color digital copying machine, an original is scanned optically and input signals B (blue), G (green) and R (red) are produced to be converted correctively into output signals Y (yellow), M (magenta) and C (cyan) for pigments (including all coloring materials such as toner, ink or ink donor film), and then reproduced as a full color image basically by outputting a combination of dot patterns of each pigment. Because equal amounts of all three pigments produce a gray shade, in actuality, an equal amount of the three components of output signals is removed (under-color removal) to avoid excessive use of the toner. However, if under-color removal is carried out, the decrease in the amount of toner causes a loss of color depth in an entire color image, and besides, the color reproducibility of simple under-color removal is not enough because the reproduction of black or gray and the reproduction of colors of a high degrees of saturation are in conflict with each other. To make up for this loss of color depth, and for producing gray tones, black pigment (K) is added in proportion to the amount of the pigments removed.
An outline of a conventional method for under-color removal in a digital color image forming apparatus proposed by the same inventor of the present invention (for example, Japanese Patent Application Unexamined Publication Hei. 2-118680) is now given.
FIG. 5 is a block diagram showing a configuration of an example of a digital color image forming apparatus.
In FIG. 5, an image input device 100 uses a CCD line sensor to read an original, performs color separation and produces the input signals B, G and R and then converts them into digital image data. An image output device 115 performs exposure and development by laser beams to reproduce the color image. From an equivalent neutral density conversion module 101 to an image output device interface 110 constitute an image processing system for the image data, where the B, G, R input signals are converted into the output signals for the pigments Y, M, C and K, and then an output signal corresponding to a developing color is selected and output at every developing cycle. In converting the input signals B, G and R into the output signals Y, M, C and K, a number of problems occur, such as how to control the balance of the colors, how to compensate for the colors corresponding to input characteristics of the image input device and output characteristics of the image output device, how to control the density and the contrast, how to control edge enhancement or unsharpness, and how to prevent moire effects, and so on.
The image input device 100 includes a CCD line sensor which scans for each of the primary colors blue, green and red at a size of 16 pixels/mm and outputs data with 24 bits (3 colors .times.8 bits; 256 levels) per pixel. The CCD sensor has 16 pixels/mm and a length of 300 mm, and filters for input signals B, G and R are installed on its upper surface. The CCD sensor scans 16 lines/mm at a scan speed of 190.5 mm/sec, and then outputs input signals on each color at a rate of about 15 million pixels/sec. The image input device 100 converts information of reflectance into information of density and further converts them into digital signals by logarithmic conversion of analog signals for B, G and R picture elements.
The image processing system inputs B, G, R input signals to the image input device 100, performs various data processing to improve the gray-scale reproducibility, and so on, and selects an output signal for a development process color from the output signals Y, M, C and K to convert it into on/off signals, and then outputs to the image output device 115. The image processing system includes, as shown in FIG. 5, an equivalent neutral density conversion module 101 converting the color signals to obtain gray tones, a color masking module 102 converting input signals B, G and R into output signals in proportion to the amounts of toner for the signals Y, M, C calculated by a matrix operation. The image processing system also includes an original size detection module 103 detecting the document size in pre-scanning and erasing platen color (frame erasing) at the time of original reading, a color conversion module 104 converting designated colors in certain areas according to area signals being input to an area image control module, an under-color removal and black generation module 105 producing an appropriate amount of black without causing muddiness and reducing the same amount of Y, M and C in proportion to the amount of produced black, and inputting signal K and signals Y, M and C according to signals indicating monochrome mode and full color mode after under-color removal is performed, and a space filter 106 having functions for controlling edge unsharpness and eliminating moire effects. Further the image processing system includes a tone reproduction control module 107 performing density control, contrast control, negative and positive inversion, color balance control to improve reproducibility, a zoom processing module 108 performing magnification and compression in the main scanning direction, a screen generator 109 converting gray-scale toner signals for process colors into on/off binary toner signals and outputting them, an image output device interface module 110, an area image control module 111 having an area generation circuit and a switch matrix, and an edit control module having an area command memory 112, a color palette video switch circuit 113 and a font buffer 114.
The tone control system comprises the parts responsible for color adjustment including an equivalent neutral density conversion module 101, a color masking module 102, an under-color removal and black generation module 105 and a tone reproduction control module 107. The equivalent neutral density module 101 corrects for spectral characteristics of the light source for the image input device and characteristics of the dichroic mirror, color characteristics of the photoelectric element, color filters and lenses, and then converts into separated color signals ENDB, ENDG and ENDR with gray balance. The color masking module 102 carries out an operation using matrices which are for example, 3.times.3, 3.times.6 or 3.times.9, based on the proportion of the gray balanced separated color signals ENDB, ENDG and ENDR output from equivalent neutral density conversion module 101, and generates data for picture elements of gray balanced toner signals for three colors ENDY, ENDM and ENDC for a full color original, and generates an intensity signal for a monochrome original. The under-color removal and black generation module 105 generates a black signal K based on the proportion of the toner signals ENDY, ENDM and ENDC, and then reduces the values of toner signals ENDY, ENDM and ENDC (that is, performs under-color removal) according to the value of black signal K generated. In generating the black signal K, the maximum and minimum values of toner signals ENDY, ENDM and ENDC are determined by a maximum and minimum value detecting circuit, and the difference between the maximum and minimum values is calculated by a subtracter, and then a value derived from the difference by a chroma function is subtracted from the minimum value by the subtracter. In under-color removal, the under-color removal function converts the generated ENDK into ENDK' and the value of ENDK' is subtracted from each value of the toner signals ENDY, ENDM and ENDC by the subtracter. For a three-color or a monochrome copy, under-color removal and black production module 105 is by-passed because generation of black and under-color removal of ENDY, ENDM and ENDC are not carried out. A tone reproduction control module 107 determines a value of picture element data to be output according to picture element data having been input based on a tone reproduction control curve, and performs the control of density, contrast and color balance, negative and positive inversion and so on by altering the tone reproduction control curve.
In brief, the tone control system firstly performs the equivalent neutral density conversion on the separated color signals input to the image input device to make them gray balanced, and then carries out color masking to generate toner signals. The gray balance method is also applied in color masking, and moreover, black generation and under-color removal of the same amount from the toner signals ENDY', ENDM' and ENDC' are performed by the under-color removal function to make each toner signal always be the same amount with respect to gray. If values of the toner signals ENDY', ENDM' and ENDC' are same, toner signals Y, M and C are controlled by the tone reproduction control so that the image output device outputs gray, thereby control of gray balance in the image data processing system is carried out and reproducibility of gray in the image output system is improved.
In a conventional digital color image forming apparatus, under-color removal processing is performed on separated color signals B, G and R, and the pigment signals Y, M, C and K for the subtractive primaries are generated. However, if under-color removal is performed at low image densities, black cannot be added so much because it causes a lack of saturation and deterioration of lightness. Therefore, above-mentioned proposal by the inventor of the present invention applies a chroma function and the under-color removal function to make it practicable to perform under-color removal at low densities.
However, it is still difficult to make up for the lack of saturation and deterioration of lightness in a pictorial image when for example black characters in a color image are to be reproduced only by black toner, or under-color removal is to be carried out even at extremely low densities. And what is worse, black is a factor in muddiness in colored portions of the pictorial image, therefore, it causes problems of deterioration in tone reproduction.